The present invention relates generally to open magnets, and more particularly to a mechanical stabilizer-tuned damper attached to an open magnet.
As is well known, a magnet coil can be made superconductive by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogenic liquid. The extreme cold reduces the resistance of the magnet coil to negligible levels, such that when a power source is initially connected to the coil to introduce a current flow through the coil, the current will continue to flow through the coil due to the negligible coil resistance even after power is removed, thereby maintaining a strong magnetic field. Known superconductive magnet designs include closed magnets and open magnets.
Closed magnets typically have a single, tubular shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electrical current in the same direction. The main coils are thus designed to create a magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet""s bore where the object to be imaged is placed.
Open magnets typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personal for surgery or other medical procedures during magnetic residence imaging. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design.
The high field open (hereinafter xe2x80x9cHFOxe2x80x9d) magnet also utilizes superconducting magnet technology to generate its magnetic field. The HFO magnet essentially rotates a cylindrical magnet to the vertical position. The rotation of the magnet by itself seems relatively inconsequential. As a result of the relatively high center of gravity of the magnet, however, the mechanical structure can move dynamically with very little mechanical force applied to the HFO magnet. In the field of magnetic resonance imaging (hereinafter xe2x80x9cMRIxe2x80x9d), the requirement for main field magnetic stability is well documented. Physical magnet mechanical motion can generate magnet field disturbances. The sharpness of an MRI image depends, in part, on the magnetic field in the imaging volume being time-constant and highly uniform. Magnet vibration can cause spatial and time domain magnetic field distortion. In the case of the HFO magnet, the high center of gravity and the relatively small magnet footprint inherently creates a mechanically unstable magnet. What is needed is a mechanical stabilizer-tuned damper to minimize the magnet motion, thus reducing the risk of image phase ghosting, by reducing the amplitude of motion at very specific vibration frequencies. The mechanical stabilizer-tuned needs to be designed to dissipate vibratory motion at a specific frequency and direction of motion. The physical magnet structure is fitted to accept a mechanical stabilizer-tuned damper. The orientation of the mechanical stabilizer-tuned damper can be either in the vertical or horizontal plane, depending upon the direction of motion to be controlled.
In an illustrative embodiment of the invention, a high field open magnet includes a first upper magnet assembly, a lower second magnet assembly, at least one nonmagnetizable support beam, and a mechanical stabilizer-tuned damper assembly mounted to the first magnet assembly. Each magnet assembly includes a longitudinally-extending axis, at least one superconductive main coil generally coaxially aligned with the axis, a vacuum enclosure enclosing the assembly""s at least one main coil. The first axis of the first upper magnet assembly is generally vertically aligned, the second lower magnet assembly is positioned generally vertically below the first assembly, and the second axis of the second assembly is generally coaxially aligned with the first axis. The at least one support beam is generally vertically aligned and has a first longitudinal end attached to the first assembly and a second end attached to the second assembly. A support skirt is provided which is generally longitudinally-extending, annularly-cylindrical and generally coaxially aligned with the second axis, the skirt having a first longitudinal end attached to the second assembly and a second longitudinal end which can be supported by a floor. The first magnet assembly is fitted to accept a mechanical stabilizer-tuned damper assembly. The mechanical stabilizer-tuned damper is designed to reduce the dynamic response of the primary structure, the magnet. A connecting device is provided which consists of a threaded rod mounted to an attachment bracket through a linear bearing and vibration mount. A series of non-magnetic and extremely low electrical conductivity plates are provided to change the mass and therefore the tuning frequency of the damper. The location and size of the mechanical stabilizer-tuned damper are determined to balance the frequency of the field oscillation and the physical room. The direction of orientation can either be in the vertical or horizontal plane, depending on the direction of motion sought to be controlled. Either single or multiple mechanical stabilizer-tuned dampers can be fitted to detune one or more displacement components, or frequencies.
Several benefits and advantages are derived from the invention. It is noted that, in a vertically-aligned HFO magnet, when the support members provide a xe2x80x9cclam-shellxe2x80x9d support for the assemblies, such assemblies are particularly subject to vibration. Such clam-shell support is a very open support providing ease of patient table access to the imaging volume and providing ease of patient positioning within the imaging volume. Engineering analysis shows that the mechanical stabilizer-tuned damper design of the invention reduces magnet vibrations in a vertically-aligned open magnet having a clam-shell support for the assemblies. The mechanical stabilizer-tuned damper mass motion is aligned to dampen the direction of motion of the magnet. The mechanical stabilizer-tuned damper has the ability to add or subtract mass to fine-tune the magnet motion frequency to the natural frequency of the tuned damper. Engineering analysis also shows that mechanical stabilizer-tuned damper assemblies fitted to the magnet structure reduce the amplitude of motion at very specific oscillatory frequencies, particularly at the dominant low-excitation frequencies to which the open magnet is susceptible. When the tuned-damper magnet motion is properly matched, the amplitude of motion will reduce by as much as one order of magnitude.